Transflective liquid crystal display device

ABSTRACT

A transflective liquid crystal display device with a reflective mode using external light and a transmissive mode using a light source comprising a light source ( 11 ) used in the transmissive mode, a liquid crystal panel ( 12 ), arranged over the light source, for operating as display element and a diffusing optical element ( 13 ), arranged over the liquid crystal panel, for having a scattering state in the reflective mode and having a non-scattering state in the transmissive mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transflective liquid crystal displaydevice, particularly to a transflective liquid crystal display devicefor preventing interferences caused by reflecting light in a reflectivemode and for improving contrast in, a transmissive mode.

2. Description of Related Art

A so-called transflective liquid crystal display device, which reflectsexternal light incident from the front side, guides it to the front sideand at the same time allows incident light from a backlight system onthe back to pass and guides it to the same front side, is entering intoa full-fledged commercial stage. This type of liquid crystal displaydevice provides effective image displays using principally externallight (ambient light) when an environment in which it is used is bright(reflective mode) and principally self-emitted light from the backlightsystem when the environment is dark (transmissive mode).

A prior art document “Development of Advanced TFT with Good Legibilityunder Any Intensity of Ambient Light” (by M. Kubo, et al., IDW' 99,Proceedings of The Sixth International Display Workshops, AMD3-4, page183-186, Dec. 1, 1999, sponsored by ITE and SID) discloses such a liquidcrystal display device.

A reflective liquid crystal display device uses a diffusive reflectiveplate to efficiently transform incident light into reflecting light.This diffusive reflective plate normally has an uneven surface, capableof regularly reflecting incident light as well as adding thediffused/reflected component on this uneven surface to, the reflectinglight. Since the transflective liquid crystal display device includes areflection area in pixels, it also uses a diffusive reflective plate.

In a transflective liquid crystal display device having a diffusivereflective plate, when incident light is reflected on the uneven surfaceof the diffusive reflective plate, different light rays in the sameoptical path direction are produced on one convex part of the unevensurface; light rays near the vertex of the convex part and light raysnear the valley of the convex part, which interfere with one anothercausing a phenomenon of iridescence.

For this reason, in order to cause the light rays in the same opticalpath direction to have different optical path directions, thetransflective liquid crystal display device having a diffusivereflective plate uses a polarizer subjected to anti-glare treatment oran adhesive glue (diffusive adhesive glue) having a diffusivecharacteristic.

However, the above described transflective liquid crystal display devicehas a problem that due to the polarizer subjected to anti-glaretreatment or the adhesive glue having a diffusive characteristic, and adiffusive adhesive glue in particular, transmissive light is diffused ina transmissive mode and contrast in the transmissive mode is therebyreduced.

The present invention has been implemented in view of the abovedescribed problem and it is an object of the present invention toprovide a transflective liquid crystal display device capable ofpreventing interferences caused by reflecting light in the reflectivemode and improving contrast in the transmissive mode.

SUMMARY OF THE INVENTION

The transflective liquid crystal display device of the present inventionis a transflective liquid crystal display device with a reflective modeusing external light and a transmissive mode using a light source,comprising a light source used in the transmissive mode, a liquidcrystal panel, arranged over the light source, for operating as displayelement and a diffusing optical element, arranged over the liquidcrystal panel, for having a scattering state in the reflective mode andhaving a non-scattering state in the transmissive mode.

According to this arrangement, the diffusing optical element functionsin a scattering state in the reflective mode, and can thereby diffuselight rays in the same optical path direction which may causesinterferences produced on an uneven surface of a diffusive reflectiveplate in the reflective mode and thereby change their optical pathdirections and avoid a phenomenon of iridescence caused by interferenceof light rays. Furthermore, the diffusing optical element also functionsin a non-scattering state in the transmissive mode, and can therebyprevent light from being diffused and prevent contrast from beingreduced in the transmissive mode.

The transflective liquid crystal display device of the present inventionpreferably comprises switch controlling means for controlling to supplythe diffusing optical element with power such that the diffusing opticalelement has a scattering state in the reflective mode and has anon-scattering state in the transmissive mode.

In the transflective liquid crystal display device of the presentinvention, the liquid crystal panel preferably has a pair of glasssubstrates sandwiching a liquid crystal layer and polarizer arranged oneach glass substrate, wherein the diffusing optical element is arrangedbetween one glass substrate and the polarizer arranged on the one glasssubstrate.

According to this arrangement, the glass substrate is not arranged on,the top layer, which eliminates the need for forming an antireflectivefilm on the surface of the glass substrate or applying antireflectivetreatment.

In the transflective liquid crystal display device of the presentinvention, the diffusing optical element preferably has a polymerdispersed liquid crystal or a polymer network liquid crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an arrangement of the transflective liquidcrystal display device according to Embodiment 1 of the presentinvention;

FIG. 2 is a sectional view showing an arrangement of the liquid crystalpanel of the transflective liquid crystal display device according toEmbodiment 1 of the present invention;

FIG. 3 is a sectional view showing an arrangement of the diffusingoptical element of the transflective liquid crystal display deviceaccording to Embodiment 1 of the present invention;

FIG. 4 is a block diagram showing an arrangement of the transflectiveliquid crystal display device according to Embodiment 1 of the presentinvention;

FIG. 5 is a view showing a switching table in the transflective liquidcrystal display device according to Embodiment 1 and Embodiment 2 of thepresent invention;

FIG. 6 is a sectional view showing an arrangement of the liquid crystalpanel of the transflective liquid crystal display device according toEmbodiment 2 of the present invention; and

FIG. 7 is a view showing another switching table in the transflectiveliquid crystal display device according to Embodiment 1 and Embodiment 2of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

With reference now to the attached drawings, embodiments of the presentinvention will be explained in detail below.

Embodiment 1

This embodiment will describe an arrangement of a diffusing opticalelement arranged over a liquid crystal panel which can be changed suchthat it has a scattering state in a reflective mode and a non-scatteringstate in a transmissive mode. FIG. 1 is a view showing an arrangement ofthe transflective liquid crystal display device according to Embodiment1 of the present invention.

The transflective liquid crystal display device shown in FIG. 1 isprincipally constructed of a backlight 11 which is a light source usedin the transmissive mode, a liquid crystal panel 12 arranged over thisbacklight 11 for functioning as a display element and a diffusingoptical element 13 which can be changed so as to have a scattering statein the reflective mode and a non-scattering state in the transmissivemode.

As the backlight 11, one used for a normal liquid crystal display devicecan be used.

As the liquid crystal panel 12, a liquid crystal panel used for amonochrome transmissive liquid crystal display device, for example, a TN(Twisted Nematic) liquid crystal panel or STN (Super TwistedNematic)-liquid crystal panel can be used. Furthermore, it is alsopossible to use an active matrix liquid crystal panel or various typesof liquid crystal panels regardless of the liquid crystal type, drivingmethod, alignment mode (for example, VA (Vertical Alignment), IPS (InPlane Switching), etc.). As the liquid crystal panel 12, for example,the liquid crystal panel whose arrangement is shown in FIG. 2 can beused.

FIG. 2 is a sectional view showing an arrangement of the liquid crystalpanel of the transflective liquid crystal display device according toEmbodiment 1 of the present invention. A transparent electrode 23 isformed on one principal surface of one glass substrate 21. As thematerial of the transparent electrode 23, for example, ITO (Indium TinOxide), a zinc oxide based material, titanium oxide based material,indium oxide-zinc oxide based material, gallium added zinc oxide basedmaterial or a p-type oxide material, etc., can be used.

A resin layer 24 on which a transmission area in pixels is patterned isformed on the transparent electrode 23. As the material of the resinlayer 24, a normal resist material such as polyimide can be used. Adiffusive reflective plate 25 is formed on the resin layer 24. As thematerial of the diffusive reflective plate 25, aluminum or silver, etc.,can be used. The diffusive reflective plate 25 is formed so as to havean uneven surface and can be formed, for example, by coarsening the basesurface by etching, etc., or by patterning photosensitive resin to formunevenness.

The resin layer 24 and diffusive reflective plate 25 can be patterned asfollows, for example. First, the resin layer is formed on thetransparent electrode 23 and then a diffusive reflective plate is formedon the resin layer. Then, a resist layer is formed on the diffusivereflective plate, patterned using a photolithographic method and thediffusive reflective plate is etched using the patterned resist layer asa mask. Then, the resin layer is etched using the patterned diffusivereflective plate as a mask. In this way, the resin layer 24 anddiffusive reflective plate 25 are formed. Here, the case where the resinlayer and diffusive reflective plate are laminated, the diffusivereflective plate and the resin layer are etched in that order and thenpatterned has been explained, but according to the present invention itis also possible to laminate and pattern the resin layer first and thenlaminate and pattern the diffusive reflective plate.

An alignment film 26 is formed on the diffusive reflective plate 25 andthe transparent electrode 23 in the transmission area. As the alignmentfilm 26, a resin material such as polyimide can be used.

A color filter 27 is formed on one principal surface of the other glasssubstrate 22. A transparent electrode 28 is formed on the color filter27 and an alignment film 29 is formed on the transparent electrode 28.As the material of the transparent electrode 28 and the alignment film29, the same material as that of the glass substrate 21 can be used.

The transparent electrodes 23 and 28 of the glass substrates 21 and 22form a matrix of scanning electrodes and signal electrodes to allowdisplays. This allows pixels to be formed in the liquid crystal panel 12as with a normal liquid crystal panel. As the method of forming thetransparent electrodes 23 and 28, a method used in manufacturing anormal liquid crystal display device, for example, a sputtering methodcan be used. As the method of forming the alignment films 26 and 29, amethod used in manufacturing a normal liquid crystal display device, forexample, a method including an application step, drying step and rubbingstep, etc., can be used.

A liquid crystal layer 32 is formed between the glass substrates 21 and22. The liquid crystal layer 32 is formed by arranging the glasssubstrates 21 and 22 whose films have been formed in such a way that thealignment films 26 and 29 face each other and injecting a liquid crystalmaterial (here, TN liquid crystal) between the glass substrates 21 and22. A polarizer 30 is arranged on the other principal surface of theglass substrate 21 and a polarizer 31 is arranged on the other principalsurface of the glass substrate 22.

For the polarizers, it is possible to use a combination of a pluralityof optical films such as a phase difference film or optical film forvisual angle compensation or a polarizer made up of a single opticalfilm.

As the diffusing optical element 13, a polymer network liquid crystaldisplay element or a polymer dispersed liquid crystal display elementcan be used. For example, the polymer network liquid crystal displayelement having an arrangement shown in FIG. 3 can be used.

FIG. 3 is a sectional view showing an arrangement of the diffusingoptical element of the transflective liquid crystal display deviceaccording to Embodiment 1 of the present invention. A transparentelectrode 43 is formed on one principal surface of one glass substrate41. Furthermore, a transparent electrode 44 is formed on one principalsurface of the other glass substrate 42. As the method of forming thetransparent electrodes 43 and 44, a method used for manufacturing anormal liquid crystal display device can be used. Furthermore, as thematerial of the transparent electrodes 43 and 44, the same material asthat used for the above described liquid crystal panel 12 can be used.As the method of forming them, the method used for manufacturing anormal liquid crystal display device can be used.

A polymer liquid crystal layer 45 is formed between the glass substrates41 and 42. The polymer liquid crystal layer 45 is sandwiched between theglass substrates 41 and 42 arranged in such a way that the transparentelectrodes 43 and 44 face each other.

As the polymer liquid crystal layer 45, a polymer network liquid crystalmade up of a network including liquid crystal molecules which extends ina polymer matrix or a polymer dispersed liquid crystal made up ofdroplets including liquid crystal molecules dispersed in a polymermatrix can be used. From the standpoint of a low driving voltage, thepolymer network liquid crystal is advantageous.

The above described liquid crystal panel 12 and diffusing opticalelement 13 are arranged as shown in FIG. 1, in the order of thebacklight 11, liquid crystal panel 12 and diffusing optical element 13.In such an arrangement, the liquid crystal panel 12 functions as adisplay element in either mode and the diffusing optical element 13 iscontrolled to have a scattering state in the reflective mode and anon-scattering state in the transmissive mode.

FIG. 4 is a block diagram showing an arrangement of the transflectiveliquid crystal display device according to Embodiment 1 of the presentinvention. This arrangement includes a control section 51 which controlsthe overall apparatus, the backlight 11, the liquid crystal panel 12, aswitching control section 52 which controls switching of voltageapplication to the optical element 13 and a power supply 53 whichsupplies power for voltage application. Furthermore, this arrangementincludes switches SW1 and SW2 for switching control. Furthermore, theswitching control section 52 includes a switching table shown in FIG. 5and performs switching control based on this switching table.

Then, the operation of the transflective liquid crystal display deviceaccording to this embodiment with the above described arrangement willbe explained.

The control section 51 automatically sets a display mode (reflectivemode or transmissive mode) based on an input from the user or situationof the environment (brightness and light quantity, etc.) and outputs themode information to the switching control section 52. In the reflectivemode, the switching control section 52 controls SW1 and SW2 in such away that SW1 and SW2 switch the power supply so as to use external lightand use the diffusing optical element 13 in a scattering state, and inthe transmissive mode, SW1 and SW2 switch the power supply so as to uselight of the backlight 11 and use the diffusing optical element 13 in anon-scattering state.

First, the case in the reflective mode will be explained. In thereflective mode, more specifically as shown in FIG. 5, SW1 is set to theOFF position to use the diffusing optical element 13 in a scatteringstate. Since external light is used in this reflective mode, no power issupplied to the backlight 11. Therefore, SW2 is also set to the OFFposition.

In such a condition, displays are performed with power supplied from thepower supply 53 to apply a voltage to the liquid crystal panel 12. Inthis case, incident light, which is external light, passes through thediffusing optical element 13, reaches the liquid crystal panel 12, isefficiently reflected by the diffusive reflective plate 25 of the liquidcrystal panel 12 to become reflecting light and passes through thediffusing optical element 13 from the liquid crystal panel 12 to theoutside. At this time, incident light is regularly reflected by thediffusive reflective plate 25 and at the same time the uneven surface ofthe diffusive reflective plate 25 adds a diffused/reflected component tothe reflecting light. When the incident light is reflected by the unevensurface of the diffusive reflective plate 25, light rays in the sameoptical path direction are produced on one convex part of the unevensurface; light rays near the vertex of the convex part and light raysnear the valley of the convex part. Since the diffusing optical element13 is in a scattering state, when these light rays in the same opticalpath direction pass through the diffusing optical element 13, they arediffused and thereby have different optical paths. In this way, thereflecting light that has passed through the diffusing optical element13 is free of interference and produces no phenomenon of iridescence. Asa result, it is possible to maintain high display quality as the liquidcrystal display device.

Then, the case in the transmissive mode will be explained. In thetransmissive mode, more specifically as shown in FIG. 5, SW1 is set tothe ON position so that the diffusing optical element 13 is set in anon-scattering state to allow the light of the backlight 11 to pass.Furthermore, since no external light is used in this transmissive mode,power is supplied to the backlight 11. Thus, SW2 is also set to the ONposition.

In this condition, displays are performed with power supplied from thepower supply 53 to apply a voltage to the liquid crystal panel 12. Inthe transmissive mode, since the diffusing optical element 13 is in ano-scattering state, the light from the backlight 11 which has passedthrough the liquid crystal panel 12 is allowed to go outside. In thisway, it is possible to prevent contrast from being reduced in thetransmissive mode. Furthermore, the transmissive mode is assumed to beused in a place with less external light, and therefore interference inthe diffusive reflective plate 25 has no influence on the displayquality.

In this way, in the transflective liquid crystal display deviceaccording to Embodiment 1, the diffusing optical element functions in ascattering state in the reflective mode, and can thereby diffuse lightrays in the same optical path direction produced on the uneven surfaceof the diffusive reflective plate in the reflective mode which may causeinterference so as to make their optical path directions differ from oneanother and avoid a phenomenon of iridescence due to interference oflight rays. Furthermore, the diffusing optical element functions in anon-scattering state in the transmissive mode, and therefore light isnot diffused in the transmissive mode and it is possible to preventcontrast from being reduced. Furthermore, in the transflective liquidcrystal display device according to Embodiment 1, the diffusing opticalelement 13 is arranged over the liquid crystal panel 12, and thereforethe polarization state of the liquid crystal panel 12 is not affected.

Embodiment 2

This embodiment will describe an arrangement with the diffusing opticalelement 13 incorporated in the liquid crystal panel 12. FIG. 6 is asectional view showing an arrangement of the liquid crystal panel of thetransflective liquid crystal display device according to Embodiment 2 ofthe present invention. In FIG. 6, the same parts as those in FIG. 2 areassigned the same reference numerals and detailed explanations thereofwill be omitted.

A diffusing optical element 13 is arranged on the other principalsurface of a glass substrate 22 of a liquid crystal panel 61 shown inFIG. 6, that is, on the principal surface opposite the principal surfaceon which a color filter 27 is provided. The arrangement of the diffusingoptical element 13 is the same as that shown in FIG. 3. A polarizer 31is arranged on the diffusing optical element 13.

The operation of the transflective liquid crystal display deviceaccording to this embodiment having the above described arrangement issubstantially the same as that of Embodiment 1. First, the case in thereflective mode will be explained. In the reflective mode, morespecifically as shown in FIG. 5, SW1 is set to the OFF position to usethe diffusing optical element 13 in a scattering state. Since externallight is used in this reflective mode, no power is supplied to thebacklight 11. Therefore, SW2 is also set to the OFF position.

In such a condition, displays are performed with power supplied from thepower supply 53 to apply a voltage to the liquid crystal panel 61. Inthis case, incident light, which is external light, passes through thediffusing optical element 13 of the liquid crystal panel 61, isefficiently reflected by the diffusive reflective plate 25 to becomereflecting light and passes through the diffusing optical element 13from the liquid crystal panel 61 to the outside. At this time, incidentlight is regularly reflected by the diffusive reflective plate 25 and atthe same time the uneven surface of the diffusive reflective plate 25adds a diffused/reflected component to the reflecting light. When theincident light is reflected by the uneven surface of the diffusivereflective plate 25, light rays in the same optical path direction areproduced on one convex part of the uneven surface; light rays near thevertex of the convex part and light rays near the valley of the convexpart. Since the diffusing optical element 13 is in a scattering state,when these light rays in the same optical path direction pass throughthe diffusing optical element 13, they are diffused and thereby havedifferent optical paths. In this way, the reflecting light that haspassed through the diffusing optical element 13 is free of interferenceand produces no phenomenon of iridescence. As a result, it is possibleto maintain high display quality as the liquid crystal display device.

Then, the case in the transmissive mode will be explained. In thetransmissive mode, more specifically as shown in FIG. 5, SW1 is set tothe ON position so that the diffusing optical element 13 is set in anon-scattering state to allow the light of the backlight 11 to pass.Furthermore, since no external light is used in this transmissive mode,power is supplied to the backlight 11. Thus, SW2 is also set to the ONposition.

In this condition, displays are performed with power supplied from thepower supply 53 to apply a voltage to the liquid crystal panel 61. Inthe transmissive mode, since the diffusing optical element 13 is in ano-scattering state, the light from the backlight 11 which has passedthrough the liquid crystal panel 61 is allowed to go outside. In thisway, it is possible to prevent contrast from being reduced in thetransmissive mode. Furthermore, external light in the transmissive modeis reduced, and therefore interference in the diffusive reflective plate25 has no influence on the display quality.

In this way, in the transflective liquid crystal display deviceaccording to Embodiment 2, the diffusing optical element functions in ascattering state in the reflective mode, and can thereby diffuse lightrays in the same optical path direction produced on the uneven surfaceof the diffusive reflective plate in the reflective mode which may causeinterference so as to make their optical path directions differ from oneanother and avoid a phenomenon of iridescence due to interference oflight rays. Furthermore, the diffusing optical element functions in anon-scattering state in the transmissive mode, and therefore light isnot diffused in the transmissive mode and it is possible to preventcontrast from being reduced. Furthermore, in the transflective liquidcrystal display device according to Embodiment 2, the glass substrate ofthe diffusing optical element 13 is not arranged on the top layer, andtherefore it is not necessary to form any antireflective film on thesurface of the glass substrate or apply antireflective treatment.

The present invention is not limited to above described Embodiments 1and 2, but can be implemented modified in various ways. For example,Embodiments 1 and 2 have described the case where a passive liquidcrystal display element is used as a liquid crystal panel used as thedisplay element, but an active matrix liquid crystal display element canalso be used in the present invention.

Furthermore, Embodiments 1 and 2 have described the case where a polymernetwork liquid crystal is used as the diffusing optical element, but thepresent invention is also applicable to a case where polymer dispersedliquid crystal is used as the diffusing optical element. Furthermore, asthe diffusing optical element in the present invention, it is possibleto use not only the diffusing optical element using the polymer networkliquid crystal or polymer dispersed liquid crystal but also a diffusingoptical element capable of electrically controlling the switchingbetween a state in which light is allowed to pass and a state in whichlight is diffused.

Furthermore, Embodiments 1 and 2 above have described the case where thediffusing optical element 13 arranged in such a way that the polymerliquid crystal layer 35 is sandwiched between the glass substrates isused, but the present invention is also applicable to a case where thediffusing optical element 13 is a film which does not use any glasssubstrate. In this case, it is possible to paste it to the liquidcrystal panel and thereby simplify the manufacturing process.

Furthermore, Embodiments 1 and 2 above have described the case where thediffusing optical element 13 is controllable so as to have a scatteringstate when no voltage is applied and have a non-scattering state(transparent) when a voltage is applied, but the present invention isalso applicable to a case where the diffusing optical element iscontrollable so as to have a non-scattering state (transparent) when novoltage is applied and have a scattering state when a voltage isapplied. In this case, as shown in FIG. 7, switching control isperformed in such a way that SW1 is set to the ON position and SW2 isset to the OFF position in the reflective mode and SW1 is set to the OFFposition and SW2 is set to the ON position in the transmissive mode. Inthis mode, power consumption can be reduced in the transmissive mode,which is preferable when importance is attached to the transmissivemode.

The present invention can provide sufficiently bright displays in boththe transmissive mode and reflective mode, and is therefore applicableto all kinds of liquid crystal display devices used in an externalenvironment such as cellular phone and PDA (information portableterminal) or liquid crystal display devices mounted on vehicles orairplanes.

As described above, the present invention provides a transflectiveliquid crystal display device with a reflective mode using externallight and a transmissive mode using a light source, comprising a lightsource used in the transmissive mode, a liquid crystal panel, arrangedover the light source, for operating as display element and a diffusingoptical element, arranged over the liquid crystal panel, for having ascattering state in a reflective mode and having a non-scattering statein a transmissive mode, and can thereby prevent interferences caused byreflecting light in the reflective mode and improve contrast in thetransmissive mode.

This application is based on Japanese Patent Application No. 2002-339552filed on Nov. 22, 2002, entire content of which is expresslyincorporated by reference herein.

1. A transflective liquid crystal display device with a reflective mode using external light and a transmissive mode using a light source comprising: a light source used in the transmissive mode; a liquid crystal panel, arranged over said light source, for operating as display element; and a diffusing optical element, arranged over said liquid crystal panel, for having a scattering state in said reflective mode and having a non-scattering state in said transmissive mode.
 2. The device according to claim 1, further comprising switch controlling means for controlling to supply said diffusing optical element with power such that said diffusing optical element has a scattering state in said reflective mode and has a non-scattering state in said transmissive mode.
 3. The device according to claim 1, wherein said liquid crystal panel has a pair of glass substrates sandwiching a liquid crystal layer and polarizer arranged on each glass substrate, wherein said diffusing optical element is arranged between one glass substrate and said polarizer arranged on said one glass substrate.
 4. The device according to claim 1, wherein said diffusing optical element has a polymer dispersed liquid crystal or a polymer network liquid crystal. 